Medical radiation treatments of deep tumors are planned based on CAT (computer aided tomography) or MRI (magnetic resonance imaging) cross-sectional views. Such views identify and locate a treatment volume relative to other major organs. A major problem occurs in translating these views to radiation treatment parameters, especially the energy of the treatment-beam particles. For example, a CAT scan reconstructs internal features by measuring the absorption of X-rays by the body from various angles. The X-rays used are of such high energy that greater than 90% are transmitted through the body without absorption.
However, radiation treatment requires that the irradiating beam stop at the bottom of the volume of the treatment region, to prevent damage to deeper organs. If particles are used for the radiation treatment (such as protons or electrons), a large correction must be applied to relate the energy loss of the particles to the X-ray absorption values of the CAT scan. Similar considerations apply to MRI scans that are converted to radiation treatment parameters.
In summary, the diagnostic phase is implemented through use of CAT or MRI scans, however the treatment phase may be done either with a radiation beam of different energy or with different types of particle beams. Stepping from the diagnostic to the treatment phase requires theoretical assumptions about how the body interacts to the various types of radiation. As an example, consider the treatment of a prostate cancer located by CAT scans. The radiation volume is identified by use of CAT images. The radiation treatment chosen involves the use of a proton beam that penetrates approximately 12 cm to the prostate tumor.
The treatment planner needs to convert the CAT images to equivalent proton energy loss, so that the initial proton energy will allow the protons to penetrate to the prostate and no further. This is done using a conversion table for deriving Hounsfield Units (HU), which relate the CAT image density to body density. From this the planner can model the treatment phase, using values of the proton energy loss.
It is important that the incident protons not penetrate further than the prostate gland, for the colon (just distal to the prostate) is much more sensitive to radiation and may be harmed by small amounts of proton irradiation. It has recently been shown that the required conversion factor, HU, from CAT scans to proton energy loss, may be inaccurate for various organs and tissue types. This is reported in "Range Precision of Therapeutic Proton Beams", B. Schaffner, Ph.D. Thesis, submitted to the Swiss Federal Institute of Technology, Zurich (Switzerland), 1997.
Further, CAT scans are rarely taken with the patient on the treatment gurney. For deep tumors, there may be significant organ motion between the time of a CAT scan and the radiation treatment, limiting the CAT scan reliability for predicting body density encountered by the treatment beam. For example, the CAT scan may show 6 cm of partially-full small intestines that the proton beam must penetrate to reach the prostate. But at the time of irradiation treatment (perhaps several days later), there may be only 5 cm of empty intestines to be penetrated. This kind of "motion" error and also errors in the HU units, may be compensated for by increasing the volume of irradiation, leading to peripheral organ damage and increased radiation burden to the patient.
Accordingly, it is an object of the invention to provide a method and apparatus for adjusting a treatment beam so as to assure a desired level of beam energy in a treatment volume.
It is a further object of the invention to provide a method and apparatus for adjusting the positioning of a treatment beam so as to assure a desired beam orientation with respect to a treatment volume.